Moisture control of feed material in systems including both combined dry crushing-and-grinding mills and wet grinding mills



March 11, 1958 D. WESTON 2,826,370

MOISTURE CONTROL OF FEED MATERIAL IN SYSTEMS INCLUDING BOTH COMBINED DRY CRUSHING-AND-GRINDING MILLS AND WET GRINDING MILLS Filed March-2, 1953 3 Sheets-Sheet l INVENTOR DAVID \NESTON jmVL +&

ATTORNEY March 11, 1958' n. WESTON 2,826,370 MOISTURE CONTROL OF FEED MATERIAL IN SYSTEMS INCLUDING BOTH COMBINED DRY CRUSHINGANDGRINDING MILLS AND WET GRINDING MILLS Filed March 2, 1953 5 Sheets-Sheet 2 INVENTOR D A VI LJEsTQN TTORNEY 1 D. WESTON March 11, 1958- MOISTURE CONTROL OF FEED. MATERIAL IN SYSTEMS INCLUDING BOTH COMBINED DRY CRUSHING-AND-GRINDING MILLS AND WET GRINDING MILLS s Sh ets-Sheet s INVENTOR DAVID W55 TON BflWW /f.

Filed March 2. 195a ATTORNEY 6 Claims. (Cl. 241-20) This invention relates to a method and apparatus for conditioning feed material for combined dry crushing and grinding material reduction mills requiring less than a predetermined percentage of moisture-in the feed supplied thereto for efiicient operation thereof.

More particularly, this invention relates to the conditioning of feed for combined dry crushing and grinding material reduction mills wherein the material itself acts at least in part as its own reduction medium. Such mills are adapted to reduce material in the dry state, and in a single operation from a relatively coarse state of subdivision to a product normally-suitable for metallurigcal treatment. As is well known, the efficiency of such mills drops off markedly when the moisture content of the feed material exceeds a certain percentage, which percentage depends to some extent on the type of material undergoing reduction. In practice, it is usual to ascertain, by testing, the maximum moisture content which can be tolerated in any particular instance and to provide suitable driers in advance of the mill to ensure that the maximum tolerable moisture content is never exceeded. As will be appreciated, however, driers represent a considerable capital expense, and the heat consumed in them adds materially to the cost per ton of material being treated.

I have now found that for many purposes it is possible to reduce the moisture content of feed material to the required extent without the use of driers in a simple, novel manner by a method which makes use of the fact that in the case of most mineral materials the moisture present occurs principally as free surface moisture on the surfaces of the individual particles of material and, therefore, the weight of moisture which may be carried by any given weight of material increases with the increase in total surface area of the material. The-ratio of total surface area to weight in any particulate material is a function of particle size, and it is largely the fines which are responsible for the moisture content of most mineral materials. Moreover, in the smaller particle sizes, owing to the effect of surface tension, the layer of surface moisture remaining on the. particles after draining increases in thickness as the particle size decreases so that the ratio of total free surface moisture retained to the total quantity of material present increases very rapidly in the fine size ranges. I have found that by washing feed material free of fines up to a given particle size a feed material may be produced which, although the surfaces of the individual particles remain wet, will not contain an overall quantity of moisture in relation to the total quantity of ore present which is sufiicient to adversely affect the operational efficiency of a material reduction mill of the type above described.

According to my invention, therefore, I provide a method of conditioning feed material for combined dry crushing and grinding material reduction mills to reduce the moisture content thereof which includes the steps of washing said material and removing therefrom, during washing, a fines'fraction containing sufficient fine material within particle size ranges capable of retaining more than ited States Patent 2,826,370 Patented Mar. 1 1, .1958

a predetermined percentage of their weight as free surface moisture to reduce the overall capacity of the remaining material for holding free surface moisture to below said predetermined percentage, andif desired .dewatering said remaining material to below said predetermined percentage.

According to one embodiment of the invention, .1 may first scalp a coarse fraction from the material, washing if necessary duringscalping to free adherent fines. In this case, the remaining material is then washed and afines fraction is removed'from it as aforesaid to-leave-a-middlings fraction which, after draining, is combined with the coarse fraction to producea conditioned feedof reduced moisture content.

In cases where the moisture content of the feed requires only slight reduction, .itmay be advantageous to .split the feed into two aliquot portions before conditioning. In this case only one aliquot portion need beconditioned, its moisture content being sufficiently reduced thereby to bring the totalmoisturecontent of the feed to-within permissible limits when the'conditioned aliquot portion minus-the removed fines is combined with theuncon'ditioned aliquot portion.

Theupresent invention afiords a most convenient and efficientmethod of dressing ores for metallurgical treat ment byusirnultaneous use of combined dry crushing and grinding, and wet grinding. The finesrfraction which is removed from the feed during the conditioning process may, if it contains values, befed directly to a wet grinding circuit comprising one or more ball mills, or the like, while the bulk .of the feed whichis in a much coarser state is fedto acombined dry crushing and grinding circuit in parallel relationship to the wetgrinding circuit. Following reduction, the products from the tworeduction circuits may be-combined for metallurgical treatment, or treated separately, depending upon the char acteristics of the material and the metallurgy employed.

My invention has particular application in connection with millsof the typedescribed inmy previous Patent No. 2,555,171v and application Serial No. 175,353 filed July 22, 1950.(now abandoned),=when operating with or without a ball charge of thetypeudescribed in my-copending application Serial No. 234,7-82-filed July 2, 1951 (now Patent No. 2,680,568). These mills will in general tolerate a moisture content up to approximately 6% by weight of the feedmaterial, although in particular instances this-figure'may vary somewhat, up or down, depending-upon the characteristics of the material which is undergoing reduction and the conditions of operation. ln 'mills of this type, normally excellent reduction efiiciency is obtained withcoarse-feed, and a particular advantage of such mills is that they are capable of taking feed containing material aslarge as 18" and reducing it toa comparatively fine product, in a single operation.

Normally, thefinematerial requiring removal forms but a small percentage of the original feed material and in many cases it-contain-s a large percentage in a fine enough state to require no further rcomminution. larger of the particles which may be so removed are comparatively finefeedto other types ofreduction units such as a ball or pebble mill. Inaccordance with the present invention, therefore, I'have found it most convenient and efficient, in cases where the ore is wet and comminution of the feed is to be followed by wet circuit metallurgy, to separate the bulk of the fine material (say of A particlesize) from the feed by washing and wet screening and to feed this separated material to a wetgrinding circuit while draining the remaining coarse material and feeding it without further drying to a dry grinding circuit comprising a material reduction mill of the type described in my above-mentioned-patentand copending applications. Depending upon the character- The istics of the mineral being treated, the products of dry grinding and wet grinding may either be combined for metallurgical treatment or may be treated separately.

Suitable apparatus according to the invention comprises scalping means with washing means arranged to wash material during its passage therethrough, wet screening means arranged to receive the material discharged from said scalping means after scalping, dewatering means arranged to receive the material held on said wet screening means and means for recombining the material scalped in said seal-ping means with the material discharged from said dewatering means. If desired, dewatering and screening may be carried out on the same screen.

The invention will be more explicitly described in conjunction with the accompanying drawing wherein:

Figure 1 illustrates a feed conditioning circuit in accordance with the present invention which is particularly adapted for use in the preparation for milling of gold ores where the comminuted ore is to be wet circuited in a 'cyanidation plant in a conventional manner, the individual components of the circuit, being of generally conventional construction, are illustrated diagrammatically.

Figure 2 is a flow sheet illustrating an alternative embodiment of the invention, and

Figure 3 is a flow sheet illustrating another alternative procedure according to the invention.

Referring particularly to Figure 1, run of mine ore, which in the circuit illustrated is minus 12" material, is fed to the circuit on the conveyor from where it is discharged partly on to the shuttle conveyor 11 and partly on to the stock pile 12. Material is fed by the shuttle conveyor 11 on to the scalping grizzly 13, and during its passage through the scalping grizzly, the material is thoroughly washed by water or barren solution issuing from the spray heads 14. In some cases where the fines are not unduly adherent on the larger particles, washing on the scalping grizzly may be omitted. The scalped material, which in the case illustrated will be plus 1 /2, is circuited directly from the scalping grizzly 13 to the conveyor 15.

The undersize of the scalping grizzly is fed to the up per end of the Wet screen 16 which is fed with additional wash water or barren solution from the spray heads 17. The wet screen 16 is designed to retain the plus 4" material while the minus 4" and the wash liquid are discharged into the head box 18. The plus 4" material is then fed to the upper end of the dewatering screen 19 where it is drained and eventually fed on to the conveyor where it is recombined with the plus 1 /2" material which was separated in the scalping grizzly 13. The liquid drained from the plus A minus 1 /2" material on the dewatering screen 19 is also fed to the head box 1.8, and the resulting pulp in the head box 18 is pumped by sludge pump 20 to a wet grinding circuit which may suitably comprise one or more ball mills and/or classifiers (not shown).

The coarse and fine fractions from the grizzly 21 are conveyed respectively to coarse and fine storage at the mill site by conveyors 22 and 23.

While the present invention has particular application as aforementioned in the conditioning of feed materials where wet metallurgy is to follow comminution, it may also be used to advantage in many completely dry circuits, particularly in cases where the run of mine or run of quarry material contains a relatively small percentage of minus /i material. In such cases, the fines removed in the washing process may either be discarded, or reduced to their final state of subdivision in a wet grinding circuit arranged in parallel with the main material reduction mill and then dried, or the thus removed fine material may be dried and recombined with the fine feed in ready storage in advance of the material reduction mill. In either case, the drying facilities required Will be much smaller than would otherwise be required,

4 and the invention results in considerable saving, both in capital cost and in heat consumption.

The alternative flow sheet shown in Figure 2 illustrates the case where washing is unnecessary on the scalping grizzly and shows how the invention may be used to feed both a wet grinding circuit and a dry crushing and grinding circuit running in parallel relation.

Referring to Figure 2, run of mine or quarry material is fed by conveyor 25 into storage hopper 26 which feeds it into the primary crusher 27. The discharge from the crusher 27 passes over scalping grizzly 28 where a coarse fraction is removed and conveyed by conveyor 29 to coarse storage hopper 30 which is adjacent the mill site.

The undersize from grizzly 28 is conveyed on conveyor 31 to storage hopper 32 which discharges onto wet screen 33 supplied with wash water or barren solution by spray heads 34.

The oversize discharge from the wet screen 33 is collected in hopper 35 and passed to the dewatering screen 36 from where it is passed to the fine feed storage hopper 37 associated with combined dry crushing and grind ing material reduction mill 38.

The undersize from wet screen 33 is collected in hopper 39 and passed together with Water extracted on dewatering screen 36 to the wet ball mill 40, which discharges into classifier 41. The overflow from classifier 41 is fed to wet metallurgy, while the sands are recirculated to the ball mill for further reduction.

The coarse feed in hopper 3t) and the fine feed in hopper 37 are fed in controlled amounts to the mill 3%, and the product of the mill is collected in wet cyclone 42 and passed to wet metallurgy either together with or separately from the overflow from classifier 41.

in carrying out the process of the invention in the manner indicated above, the primary crusher 27 will be set to produce a predetermined minimum percentage of plus 3" material, and the feeding of the mill 3% will preferably be carried out substantially in accordance with the teachings of my copending application Serial No. 203,861 (now Patent No. 2,729,397).

The procedure illustrated by the flow sheet in Figure 3 is one which may be used to particular advantage where the run of mine material contains very little material in size ranges intermediate the fines and the coarse and where a substantial percentage of the fines require no reduction prior to metallurgical treatment.

In the flow sheet illustrated, it is assumed that the original material requires to have its moisture content reduced only to a relatively small extent in order to be suitable as feed for a combined dry crushing and grinding material reduction mill.

Referring particularly to Figure 3, run of mine or run of quarry material is fed to splitter 60 by the conveyor 51. The splitter en divides the total feed discharging one aliquot portion into hopper 62 and another aliquot portion into hopper 63. The material which is collected in hopper 62 is passed directly to a grizzly 4-4 where it is divided into a coarse fraction and a fine fraction which pass into coarse and fine feed storage bins 45 and 46 respectively.

The aliquot portion discharged into hopper 63 is discharged on to the scalping grizzly 47 where it is washed by wa.er or barren solution issuing from the spray heads 43 while the scalping operation is being carried out. The oversize from the scalping grizzly 47 is dewatered on screen 49 and then passed by conveyor 50 into the coarse feed bin 4-5 where it is combined with the coarse fraction of the aliquot portion from the hopper 62.

The undersize from the scalping grizzly 47 is passed to the wet screen 51 which is supplied with water or barren solution from the spray heads 52 and a fine fraction requiring no reduction before metallurgical treatment is discharged on to conveyor 53 into hopper 54 from where it is fed directly to metallurgical treatment. The oversize from wet screen 51 is dewatered on screen asaasvo 55 from whence it is discharged on to conveyor 56 which carries it to the fine feed storage bin 46 where it is combined with the fine fraction of the aliquot portion discharged from hopper 62.

The coarse and fine storage bins 45 and 46 feed coarse and fine feed respectively at a controlled rate and in controlled portions to the combined dry crushing and grinding material reduction mill 57 from which the product is fed to metallurgical treatment together with the discharge from hopper 54.

From the foregoing, it will be apparent that the present invention not only makes possible the application of dry crushing and grinding to Wet minerals without the need for driers, but that it also provides a feed conditioning process of great versatility which may be modified as the occasion demands to meet a wide variety of conditions and take advantage of favourable factors which may exist in any particular instance.

As will be apparent, however, the present invention is not intended to be used in connection with the conditioning of feed materials of a porous or absorptive character, nor in the case of materials which are appreciably soluble.

WhatI claim as my invention is:

1. In the conditioning of material for use in feeding a combined dry crushing and grinding material reduction mill to reduce the moisture content thereof the steps which consist in; washing said material; removing from said material, during washing, a fines fraction leaving behind a coarse fraction, said fines fraction containing sufficient fine material within particle size ranges capable of retaining more than a predetermined percentage of their weight of free surface moisture to reduce the overall capacity of the remaining material for holding free surface moisture to a value below said predetermined percentage; dewatering said coarse fraction to below said predetermined moisture content; passing said coarse fraction to a combined dry crushing and grinding mill Without further drying thereof and reducing said fines fraction by wet grinding.

2. A method of dressing ore for metallurgical treatment comprising; scalping a coarse fraction from said ore; washing the remaining ore and removing therefrom during washing a fines fraction containing sufficient material within particle size ranges capable of retaining more than a predetermined percentage of their weight of free surface moisture to leave a middlings fraction having a. capacity for holding free surface moisture which is below said predetermined percentage; combining said middlings fraction with said coarse fraction and reducing them by combined dry crushing and grinding; and reducing said fines fraction by wet grinding.

3. A method of dressing ore for metallurgical treatment comprising; scalping a coarse fraction from said ore; washing the remaining ore and removing therefrom during washing a fines fraction containing sufficient material within particle size ranges capable of retaining more than a predetermined percentage of their weight of free surface moisture to leave a middlings fraction having a capacity for holding free surface moisture which is below said predetermined percentage; dewatering said middlings fraction to below said predetermined percentage moisture; combining said middlings fraction with said coarse fraction and reducing them by combined dry crushing and grinding; and reducing said fines fraction by wet grinding.

4. A method as defined in claim 3 wherein water used during washing is combined with said fine fraction to form a pulp suitable as feed for a wet grinding circuit.

5. In the conditioning of material for use in feeding a combined crushing and grinding material reduction mill to reduce the moisture content thereof the steps comprising, separating the material into a coarse and an undersize fraction, washing said undersize fraction and removing from it during washing a fines fraction containing sufficient fine material within particle size ranges capable of retaining more than a predetermined percentage of their weight of free surface moisture to leave a middlings fraction of reduced moisture-holding capacity which when combined with said coarse fraction will produce a feed material having an overall moisture-holding capacity below a limit defining the moisture tolerance of the combined crushing and grinding mill, combining said coarse and middlings fractions and passing said combined fractions to the combined dry crushing and grinding mill without further drying thereof with the fines fraction passing to wet grinding.

6. In the conditioning of material for use in feeding a combined crushing and grinding material reduction mill to reduce the moisture content thereof the steps comprising, separating the material into a coarse and an undersize fraction, washing said undersize fraction and removing from it during washing a fines fraction containing sufficient fine material within particle size ranges capable of retaining more than a predetermined percentage of their weight of free surface moisture to leave a middlings fraction having a capacity of holding free surface moisture below said predetermined percentage, dewatering said middlings fraction, combining said coarse and middlings fractions and passing them to a combined dry crushing and grinding mill without further drying thereof, and reducing said fines fraction by wet grinding.

References Cited in the file of this patent UNITED STATES PATENTS 719,942 Hermann Feb. 3, 1903 947,599 Ransome Ian. 25, 1910 1,013,100 Anderson Jan. 2, 1912 1,461,067 Moser July 10, 1923 2,453,293 Aurilio et a1. Nov. 9, 1948 FOREIGN PATENTS 15,789 Great Britain of 1888 117,708 Australia Nov. 2, 1943 OTHER REFERENCES Slurry Plant Uses Vibrating Screen for Dewatering," from Coal Age, March 1945, pages and 96. 

